This application relates to agricultural plastic articles and, more particularly, to degradable mulch film comprising slowly released multi-nutrients in a matrix of thermoplastic resin material.
Agricultural mulch films are used extensively throughout the world because of the many advantages they provide, including retarded weed growth, increased soil retention of moisture and heat, and reduced soil erosion by winds and rain. Such mulching films also enhance soil structure by preventing soil crusting and soil compaction.
Polyethylene film, both in a transparent state and an opaque state, has been the most common plastic mulch to be used. However, polyethylene film is not degradable and it must be removed from the field and be burned, buried or otherwise discarded at the end of each fruiting season. Because the removal and burying or burning of plastic mulch is both costly and has an adverse effect on the ecology, the need for a plastic mulch that would decompose at the end of a growing season recently has become apparent. This need has led to the development of a variety of biodegradable and photodegradable mulch films. For example, Otey et al., U.S. Pat. No. 3,949,145 developed a biodegradable plastic film from starch, polyvinyl alcohol and glycerol, which film is covered with a water-resistant resin coating to prevent premature degradation. The water-resistant resin coating is prepared from a water-resistant resin, such as plasticized polyvinyl chloride, and a polyol-toluene diisocyanate prepolymer bonding agent.
Another biodegradable mulch film, disclosed by Clendinning, U.S. Pat. No. 3,929,937, is fabricated from a blend of a particulate addition polymer, such as polyethylene, in a matrix of biodegradable thermoplastic oxyalkanoyl polymer, such as .epsilon.-caprolactone homopolymer. Additional ingredients can be included in the blends. Such additional ingredients include naturally occurring biodegradable products, e.g., tree bark, sawdust, peat moss, cotton seed hulls, and the like, fibrous and non-fibrous fillers, e.g., talc, limestone, bentonite, asbestos, and the like, plant nutrients, fertilizers, insecticides, pesticides, and the like.
In addition to the films disclosed by Otey et al. (U.S. Pat. No. 3,949,145 discussed above), water sensitive agriculturally useful polyvinyl alcohol products have been disclosed by Grano, Jr., U.S. Pat. No. 4,063,919, and Iwasyk et al., U.S. Pat. No. 3,387,405. In Grano, Jr., there is disclosed a plastic fertilizer rod composition comprising about 100 parts of a polyvinyl alcohol, about 0 to 20 parts of a plasticizer and about 10 to 350 parts of a fertilizer. The rods generally are produced in diameters of from about 1/8 to about 1 inch or larger and are cut to a length of 1 to 6 inches or longer as desired.
In Iwasyk et al., there is disclosed a continuous foam mulch which is produced by applying and substantially simultaneously gelling a fluid aqueous foam which comprises at least 0.5 weight percent of dissolved polyvinyl alcohol and at least 50 percent, based on the weight of the polyvinyl alcohol, of emulsified asphalt or wax. The foams disclosed in Iwasyk et al. are intended for use as a continuous mulch film. However, it is evident that such foams lack the mechanical integrity of plastic films of the type disclosed, for example, in Otey et al. or Clendinning.
Still other agricultural mulch films have been disclosed by Vigneault et al., U.S. Pat. No. 3,274,731, and Kane, U.S. Pat. No. 3,384,993. Vigneault et al., for example, discloses a mulch film comprised of at least one copolymer consisting essentially of ethylene and an ethylenically unsaturated carboxylic acid monomer such as acrylic acid, methacrylic acid, fumaric acid, maleic acid and the like. Vigneault et al. suggests that their films might also contain nutrients such as ammonia or phosphate-containing additives, and other fillers for various mulching uses. Kane also discloses a plastic mulch film that may include solid water-soluble nutrients such as ammonium nitrate, potassium chloride, potassium nitrate, potassium sulfate and urea. However, the only plastic mulch films disclosed by Kane are comprised of polyethylene, polypropylene, cellulose acetate, cellulose acetate butyrate, polyvinyl chloride acetate, styrene acrylonitrite, Surlyn A, and the like. These plastic materials are not soluble in water and must, therefore, rely on some other mechanism to impart adequate degradability so that they may be used as an agricultural mulch film that does not have to be removed from the fields after the growing season. One such degrading mechanism is disclosed by Newland et al., U.S. Pat. No. 3,454,510. In that patent, it is taught to include a pro-oxidant in a water-insoluble polyolefin film, such as polyethylene, polypropylene or poly(butene-1). Among the pro-oxidants that are disclosed are certain metal acetyl acetonates, metal alkyl benzoylacetates, metal acetyl acetonates, metal stearates and metal oleates.
While the above-discussed mulch films, and others, have been used with some degree of success, no plastic mulching film disclosed to date has been found to be completely satisfactory in the sense that it provides an adequate balance of the important properties needed for a good mulch film. These properties include, for example, (1) good mechanical properties, (2) clarity, (3) the ability to retard weed growth without the incorporation of herbicides or black pigments, (4) degradability, (5) the incorporation of nutritional materials, (6) slow release characteristics, and (7) safety to the environment and ecology by excluding the addition of toxic chemicals.